You've gotten your hands on a shiny new time machine, and you decide that your first order of business is to travel back in time and kill Hitler. But killing Hitler and preventing the Holocaust isn't quite as simple as it sounds. Here's why.

His Life is a Fixed Event in Time

"No Time Like the Past" (The Twilight Zone): Paul Driscoll is a well-meaning but ineffectual time traveler. Not only does he fail to kill Hitler (thanks to the intervention of a suspicious maid), he also fails to warn the Hiroshima police about the atomic bomb and fails to keep the Lusitania from being torpedoed. It turns out that he is unable to change past events, and, when he does effect events in the past, it is only as part of a predestination paradox.

He's More Clever Than You'd Think

I Killed Adolph Hitler by Jason: When a depressed hitman is contracted to go back in time and kill Hitler, the Fuhrer gets the better of him, stealing his time machine and leaving the hitman in the past to wait and plan his revenge.

Misfits, Series Three, Episode Four: If you're going to go back in time and kill Hitler, make sure you finish the job. When Friedrich Hirsch uses the time reversal power to confront the Nazi leader, Hitler overpowers and kills him. The larger problem: Hirsch was carrying his smartphone when he launched his ill-fated mission. Hitler finds the phone and quickly advances Nazi technology by sixty years. Oops.

You're Actually Part of a Predestination Paradox

"The Primal Solution" by Eric Norden: An elderly Holocaust survivor discovers a method of mental time travel and seeks to undo the horrors he witnessed in his youth by possessing the body of the young Hitler. He humiliates the young Austrian, then tries to goad Hitler into suicide, but Hitler regains control of his body before the deed can be accomplished. The young Hitler is so haunted by the encounter with the Jewish man's mind that he resolves that he can only find peace by exterminating the entire Jewish people.

Cradle of Darkness (The Twilight Zone): Katherine Heigl travels to 1889 Austria in order to kill the infant Hitler. She succeeds in killing the baby by jumping into a river with it, but Adolph's mother buys another baby and raises it as her own. And that baby grows into the Adolph Hitler that Heigl's character set out to kill.

His Guards Are Used to Dealing with Time Traveling Assassins

Subnormality: In this strip of the webcomic Subnormality, we learn that Hitler's guards are actually quite adept at killing time travelers before they get to Hitler. And all those attempts have got to make them wonder: should they be worried that they're on the wrong side of history?

You'll Be Thwarted By Other Time Travelers

"The Savage Time" (Justice League): The supervillain Vandal Savage travels back in time and places Hitler in cryogenic storage not to prevent the horrors of the Holocaust, but to assume control over the Nazi party and continue its regime into the present day. To reset the timeline, the Justice League travels back in time to remove Savage from power and have Hitler reinstated.

Midnighter: In the first arc of the Midnighter solo series, a man named Paulus claims to have replaced Midnighter's secondary heard with a bomb, which he will detonate unless Midnighter goes back in time and kills Hitler. Midnighter does try to kill Hitler as a young German soldier, but he is stopped by time police from the 95th Century.

Days of Cain by JR Dunn: A group of rebel time agents seek to undo one of humanity's greatest atrocities by killing Hitler, or, barring that, dismantling the death camps from within. But a society of time agents known as the Moiety is determined to preserve a certain version of the timeline of any cost, giving their agent Gasper James the unenviable task of ensuring the Holocaust goes forward.

"Wikihistory" by Desmond Warzel: One of the bylaws of the International Association of Time Travelers states that you can't kill Hitler. The problem is, everybody kills Hitler on their first trip. This leaves more experienced time travelers the onerous task of undoing the historical edits of n00bs.

Doctor Who, "Let's Kill Hitler": Mels, better known as young River Song, gets takes the Doctor hostage and decides that it would be fun to kill the Führer. As it turns out, though, another group of time travelers got there first. The Teselecta has shown up to punish Hitler before his death, but just as the crew realizes they've shown up too early in Hitler's timeline, the TARDIS comes crashing in, apparently saving Hitler. As a bonus, Hitler tries to shoot the Teselecta and ends up hitting Mels instead, triggering her regeneration into River Song. At least Rory gets to punch the lout in the face before locking him in a cupboard.

Killing Him Just Brings About a Potentially Worse Future

Making History by Stephen Fry: A history student and a physics professor manage to send a permanent male contraceptive pill back in time where Hitler's father will consume it, ensuring Hitler will never be born. But without Hitler, the Nazi party is ripe for the leadership of Rudolph Gloder, who shares Hitler's genocidal agenda, but is far more efficient, stable, patient, and charismatic. Free from Hitler's personality flaws, Gloder was able to take over all of Europe, so that, in the alternate present day, an extremely conservative US is in a cold war with the Nazis.

Command & Conquer: Red Alert: In the video game, Albert Einstein invents a time machine, which he uses to go back in time and deleted Hitler from time before he could rise to power. But, without a strong Germany, a Stalin-led Soviet Union invades Europe, and eventually the United States as well.

It's All Just a Dream

"The Man Who Dreamed the World" (The Fantastic Four): When She-Hulk, the Invisible Woman, the Human Torch, and Nick Fury mysteriously find themselves in the year 1936, Fury decides he may as well kill Hitler and slips off to Germany. Although the other three attempt to stop him, but just as the Invisible Woman seems to have talked him out of it, Fury shoots Hitler, killing him. But, it turns out that they had been in the dream of a coma victim, who snapped back to reality when Fury altered his dream timeline by killing Hitler. The actual timeline remains intact, leaving Fury in a less than happy mood.